45 C.F.R. § 1216.1-3 – Policy.

(a) Volunteers enrolled or participating in programs referred to in paragraphs (a) and (b) of § 1216.1-2 may not perform any services or duties or engage in activities which would otherwise be performed by an employed worker as part of his assigned duties as an employee.
(b) Volunteer referred to in paragraph (a) of this section may not perform any services or duties or engage in activities which will supplant the hiring of employed workers. This prohibition is violated if, prior to engaging a volunteer, an agency or organization referred to in § 1216.1-2(c) had intended to hire a person to undertake all or a substantial part of the services, duties, or other activities to be provided by the volunteer.
(c) Volunteers referred to in paragraph (a) of this section may not perform any services or duties or engage in activities which result in the displacement of employed workers. Such volunteers may not perform services or duties which have been performed by or were assigned to, any of the following:
1. (1) Presently employed workers,
2. (2) Employees who recently resigned or were discharged,
3. (3) Employees who are on leave (terminal, temporary, vacation, emergency, or sick), or
4. (4) Employees who are on strike or who are being locked out.
(d) Volunteers referred to in paragraph (a) of this section may not perform any services or duties or engage in activities which impair existing contracts for service. This prohibition is violated if a contract for services is modified or cancelled because an agency or organization referred to in § 1216.1-2(b) engages a volunteer to provide or perform all or a substantial part of any services, duties, or other activities set forth in such contract. The term “contract for services” includes but is not limited to contracts, understandings and arrangements, either written or oral, to provide professional, managerial, technical, or administrative services.
(e) Agencies and organizations referred to in § 1216.1-2(b) are prohibited from assigning or permitting volunteers referred to in § 1216.1-2(a) to perform any services or duties or engage in any activities prohibited by paragraphs (a) through (d) of this section.